cancers Article Rare Stochastic Expression of O6-Methylguanine- DNA Methyltransferase (MGMT) in MGMT-Negative Melanoma Cells Determines Immediate Emergence of Drug-Resistant Populations upon Treatment with Temozolomide In Vitro and In Vivo Thomas C. Chen 1,2, Nymph Chan 1, Radu O. Minea 1, Hannah Hartman 3, Florence M. Hofman 2 and Axel H. Schönthal 3,* 1 Department of Neurosurgery, Keck School of Medicine, University of Southern California, Los Angeles, CA 90089, USA;
[email protected] (T.C.C.);
[email protected] (N.C.);
[email protected] (R.O.M.) 2 Department of Pathology, Keck School of Medicine, University of Southern California, Los Angeles, CA 90089, USA;
[email protected] 3 Department of Molecular Microbiology & Immunology, Keck School of Medicine, University of Southern California, Los Angeles, CA 90089, USA;
[email protected] * Correspondence:
[email protected]; Tel.: +1-323-442-1730 Received: 8 August 2018; Accepted: 26 September 2018; Published: 28 September 2018 Abstract: The chemotherapeutic agent temozolomide (TMZ) kills tumor cells preferentially via alkylation of the O6-position of guanine. However, cells that express the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT), or harbor deficient DNA mismatch repair (MMR) function, are profoundly resistant to this drug. TMZ is in clinical use for melanoma, but objective response rates are low, even when TMZ is combined with O6-benzylguanine (O6BG), a potent MGMT inhibitor. We used in vitro and in vivo models of melanoma to characterize the early events leading to cellular TMZ resistance. Melanoma cell lines were exposed to a single treatment with TMZ, at physiologically relevant concentrations, in the absence or presence of O6BG.